It can be sort of sexy... as long as it's done right. If a guy decides to have long hair, he should at least take the time to maintain it. If he doesn't wash it, and brush it periodically, he'll begin to look like a dirty old hippie... and of coarse, that's not very attractive.
I'm a guy and while I don't know about the attractive thing. I like my hair to be rather long because I like to tuck it behind my ears when I'm in deep thought and not being able to kinda throws me off.